1. Field of the Invention
The present invention relates to movable access points and repeaters in a wireless communications network for minimizing coverage and capacity constraints in the network. More particularly, the present invention relates to constantly moveable access points of a core network of an ad-hoc wireless packet-switched data communications network, which provide user terminals with access to the core network and also are capable of functioning as repeaters between user terminals to enhance the self healing capabilities of the ad-hoc network.
2. Description of the Related Art
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”.
Specifically, a terrestrial cellular network or other type of conventional specialized mobile radio (SMR) system includes a plurality of interconnected stationary base stations that are distributed geographically at designated locations throughout the service area. Each stationary base station includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from user terminals, such as wireless telephones, located in its coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, the transceiver and user terminals transmit and receive the data packets in multiplexed format, such as quad-division multiple access (QDMA) format, time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base station to communicate simultaneously with several user terminals in its coverage area.
Each base station is also connected to one or more gateways that enable communication between the base station and other networks, such as the Internet and the public switched telephone network (PSTN). Accordingly, the base stations in the network enable the user terminals to communicate with each other, as well as with other destinations, such as telephony devices, in the PSTN.
Because each base station is stationary and can only handle a limited amount of communications signal traffic from the user terminals at any given time, the coverage area of a base station can vary depending on the amount of traffic that the base station is expected to experience. For example, the coverage area of a base station can be set to several miles in diameter in sparsely populated regions, such as rural regions having light wireless traffic, and can be set to less than a mile in diameter in densely populated regions, such as major metropolitan areas having heavy wireless traffic. The wireless communications network therefore must employ many stationary base stations in heavily populated metropolitan areas in order for the network to adequately service the user terminals in those regions.
The problem of providing an adequate number of base stations for a region is also exacerbated in areas where heavy commuting traffic occurs. For example, in metropolitan areas, millions of commuters may travel on specific stretches of highways during the morning and evening rush hour periods. However, at all other times of the day and on weekends, these stretches of highway may receive only a modest amount of traffic. Accordingly, it is impractical to deploy numerous base stations in these commuting areas simply to accommodate the few hours of increased activity per weekday. The number of base stations that can be deployed may also be limited due to space and zoning constraints. Hence, due to the lack of adequate base stations in these commuting areas, the network may be incapable of adequately servicing its subscribers during these busy commuting hours of the workday.
Other variables, such as the distance between a user terminal and a base station it is trying to access, interference in the base station's coverage area, the existence of structures such as buildings which block the line of sight (LOS) path between a user terminal and a base station, as well as regulatory and site restrictions, also create holes in the coverage areas which adversely affect the capacity capabilities of the network. Users become aware of such holes because they either lose service or get very poor service in a particular location. In certain instances, in order for a user terminal to greatly improve its service, the user terminal may need only move a few feet, for example, from behind a building blocking the user terminal's LOS with a base station to a position in the network that has adequate coverage and capacity capabilities.
Attempts to improve the coverage area provided by a base station include providing a rotating antenna at the fixed base station site. U.S. Pat. No. 6,222,504, incorporated herein by reference, discloses techniques for reorienting an antenna of a fixed base station. Although this solution may somewhat enhance the ability of a base station to service its coverage area, the coverage area itself remains fixed because the base station remains fixed. Another solution can be to make the base station towers mobile by making them airborne, such as on aircraft or on movable tracks. However, these solutions are difficult to deploy because of the size, cost and complexity of traditional base station equipment.
As can be further appreciated by one skilled in the art, it is also common for a mobile user terminal to travel between different base station coverage areas during use, that is, during a single telephone call. When this occurs, the base station whose coverage area the user terminal is leaving must transfer or “handoff” the user terminal to the base station whose coverage area the user terminal is entering, so that the latter base station can become the base station via which the user terminal and network continue to communicate. In densely populated areas having many base stations with small coverage areas, this handoff process may need to occur several times during a short period of time as the user terminal travels between the different coverage areas. However, in regions such as high traffic commuting regions having an inadequate number of base stations, more user terminals are competing for access to a base station within their coverage area. Accordingly, the number of lost or dropped calls that may occur during the handoff process can be increased due to the lack of adequate base station accessibility.
Many techniques have been developed using the circuit-switched cellular infrastructure to minimize data packet loss during handoff while also minimizing overhead necessary to successfully perform the handoff. For example, a technique known as “hard handoff” refers to a break-before-make technique where the original connection is dropped before the new connection is established. On the other hand, “soft handoff” is a make-before-break technique that maintains multiple simultaneous connections to the user terminal during handoff, and only drops the original connection after the new connection is established. Examples of soft handoff techniques are described in a publication by Wong et al. entitled “Soft Handoffs in CDMA Mobile Systems”, IEEE Personal Communications, December 1997, pp. 6–17, in a publication by Wong et al. entitled “A Pattern Recognition System for Handoff Algorithms”, IEEE Journal on Selected Areas in Communications, Vol. 18, No. 7, July 2000, pp. 1301–1312, and in TIA document TIA/EIA-95-B entitled “Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems”, Feb. 1, 1999, the entire contents of each of these documents being incorporated herein by reference.
With the arrival of the Internet in recent years, some wireless communications networks have moved away from the use of conventional cellular networks and their associated circuit switched routing techniques to improve voice communications services. Each mobile user terminal, telephony device, and any other device capable of communicating with the communications network, has a unique Internet Protocol (IP) address that uniquely identifies it from all other devices. A communications network employing IP sends data between destination points in digital form in discrete packets, rather than in the traditional circuit-committed protocols of the PSTN. Each of the data packets includes the sender's IP address as well as the intended receiver's IP address.
When a wireless user terminal, for example, transmits voice data to a base station of the communications network acting as the access point for the user terminal, a router associated with the base station reads the receiver IP address in the data packet. Each router includes a table of routing information, such as IP addresses of the devices local to the router, available routes, and so on. If the router recognizes from the receiver IP address that the data packet is intended for a telephony device in its immediate neighborhood or domain, the router forwards the data packet to that telephony device. However, if the router does not recognize the IP address as belonging to such a telephony device, the router forwards the data packet to an appropriate adjacent gateway in, for example, the Internet. The router of that gateway then reads the receiver IP address in the data packet, and either delivers the data packet to the appropriate telephony device in its domain, or forwards the data packet to another gateway. Once a router in a gateway recognizes the receiver IP address as belonging to a telephony device in its domain, the router in that gateway delivers the data packet to that telephony device.
It can be also noted that the use of IP to route data packets in a communications network enables the network to handle data other than voice data. For example, such IP techniques can be used to expand the versatility of the network to communicate audio, video or multimedia data between user terminals. Such networks can be configured as packet-switched data networks, as opposed to traditional circuit switched networks. A communications network employing a packet-switched core network is described, for example, in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, the entire content of which is incorporated herein by reference. An example of a wireless local area network (LAN) having mobility is set forth in IEEE Standard 802.11, Aug. 20, 1999, the entire content of which is incorporated herein by reference.
As in traditional cellular communications networks, wireless user terminals in a wireless communications network employing IP can be mobile, and can thus periodically change their access point to the network. Also, wireless user terminals can move outside their “home” network and become temporarily affiliated with a foreign network, and thus communicate via an access point on that foreign network.
Hence, as with traditional cellular communications networks, similar problems can arise in these packet-switched data networks which service mobile user terminals. That is, these types of networks can also experience an increased occurrence of dropped or lost data communications for mobile user terminals, and the inability of mobile user terminals to adequately access the network, if the number of access points deployed in high traffic areas is inadequate. U.S. Pat. No. 6,222,463, incorporated herein by reference, discloses a device that can be placed in vehicles to provide information about the vehicle to fixed roadside base stations to enable the base stations to track the vehicles. However, this patent does not provide any suitable solution for eliminating the problems discussed above relating to holes in the coverage areas of traditional cellular networks or packet-switched data networks. Accordingly, a need exists for a packet-switched data network having improved coverage and capacity capabilities, and a minimal amount of holes in its coverage area.